1. Field of the Invention
The present invention relates to a derailleur for bicycle, and more particularly, to an electronic front derailleur structure for bicycle.
2. Description of Related Art
As in bicycle structures, generally, front derailleur is fixed on the bicycle frame, corresponding to the position of a front sprocket. This allows the front derailleur to move the chain that meshed with the front sprocket and a rear sprocket to other target sprocket having predetermined size and number of teeth, in order to achieve the function of gear shifting.
Conventional front derailleur is manually operated. It is controlled by a control lever disposed on the bicycle handle through a cable, whereas the user may press the control lever, creating an upward or downward movement of the front derailleur, to move the chain towards the target sprocket, thereby completing the gear-shifting function.
However, with bicycles becoming a more advanced mode for transportation and sport, manually-operated gear-shifting structure is gradually being eliminated and replaced by electronic gear-shifting structure. The electronic front derailleur structure is driven by electrical motor, shifting the derailleur to allow the bicycle chain to move to a different sprocket, thereby achieving the gear-shifting function.
According to the prior art, FIG. 1 illustrates U.S. Pat. No. 6,679,797 B2, “Front derailleur for bicycle with electrical motor and gear reducer,” which disclosed a front bicycle derailleur comprising a supporting body 2, a band 3 of ring shape, a connecting member 6, an articulated arm 7 and a chain guide 5. The band 3 is pivotally connected with the supporting body 2, and is fastened to a bicycle frame 4. The supporting body 2 includes a penetrating shaft 10 pivotally attached to the articulated arm 7. One end of the articulated arm 7 is pivotally attached to the chain guide 5. The chain guide 5 has a groove receiving a bicycle chain. The other end of the articulated arm 7 has a fan-shaped sprocket 12; tooth part 19 of the sprocket 12 meshes with a worm screw 18. One end of the worm screw 18 is connected to an electrical motor 15. One end of the connecting member 6 is connected to the chain guide 5, and the other end thereof is connected to the supporting body 2.
When in use, the electrical motor 15 drives the worm screw 18 to rotate, allowing the worm screw 18 to mesh with the tooth part 19 and further driving the sprocket 12 to rotate around the center of the penetrating shaft 10. This allows the chain guide 5 connected to the articulated arm 7 to move upwards and downwards, and the connecting member 6 connecting the chain guide 5 and the supporting body 2 may pull the chain guide 5 towards or away from the bicycle frame 4, enabling the chain guide 5 to move the chain to the target sprocket A or B, thereby achieving the gear-shifting function.
In operation, while riding on a bicycle, shock and impact caused by all kinds of road-riding situations may occur frequently. In particular, when the chain guide 5 is impacted, the impact force is transmitted to the articulated arm 7 and the connecting member 6, through the sprocket 12, and then impact on the worm screw 18; this stops the worm screw 18 from being meshed with the tooth part 19 of the articulated arm 7, thereby causing the problem of jammed or imprecise gear transmission. The shock may even damage the electrical motor 15, causing the problem of jammed or inaccurate chain movement during gear shifting process.